Method for treating filaments of ionization gauges



Patented Aug. 27, 1 946 ME HQ Foa' TREATING FIDAMENTS. F IONIZATIQN GAUG S I Y poration,oflGalifornia;fz

This: invention: is con'cerne gaugesjprovided with a filament; employed: as an electronvsource. a The invention ;is';.sparticular-ly Y applicableto the mass :spectrometer iz(which .isxao typeofion ization gauge), gandiprovidesmeans for eliminating gas-sensitivity .in' filaments employed in spectrometers and other gauges *as electron emission-elements. p 2

In vacuum tube techniques itiiswcustomaryto treat metallicfilaments say tungsten filaments, with hydrogen or other-"reducing gases prior to using the filaments as an electron source. Thus it is customary to heat the filament in an atmosphere of hydrogen to a cherry red prior to evacuation of the Vacuum tube containing the filament. This procedure, although satisfactory from many standpoints, causes the filament to become gassensitive in the sense that the introduction of gas into the region of the filament brings about (1) a variation in the intensity of the electron,

emission of the filament a variation in the density of the electron emission, i. e. in the cross section of an emitted electron beam. Both of these phenomena are objectionable in ionization gauges and particularly in mass spectrometers, because each introduction of gas changes the calibration of the instrument.

As a result of our investigations, we have discovered that metallic filaments and particularly tungsten filaments can be rendered free of gassensitivity by treating them with a liquid hydrocarbon prior to evacuation and heating. Preferably the hydrocarbon is an aromatic compound. For example, a pure tungsten filament can be treated with a liquid parafiin hydrocarbon or with benzene (CsHs) or other liquid ring compound prior to being placed in the envelope of a mass spectrometer. If the filament is then subjected to heating to a red heat after evacuation to a low pressure, say 5 10 mm. of mercury, it will be free from gas-sensitivity under most conditions.

Our invention will be more thoroughly understood in the light of the following detailed description of the presently preferred practice:

In accordance with this practice, the filament is thoroughly washed in a liquid hydrocarbon, say in benzene. Subsequently it is placed in the envelope of the spectrometer and the envelope is then heated and simultaneously evacuated, the heating serving to drive occluded substances from the wall of the envelope and from the metal and other parts of the spectrometer enclosed in the envelope. When the pressure in the envelope has been reduced to a low value, say 5X 10- mm. Hg,

: heating of the filament 1 eral gases.

gaseous oxides of carbon and hydrocarbon vapors causes a formationof a stable layer of some compound of the metal (say tungsten) with carbon" and oxygen on the surface of the filament and that this layer acts as an armor to prevent carbonization of the filament and development of tungsten carbide at depth therein, andQthat in some manner this prevents gas sensitivity by preventing absorption'of gases on the filament. Whatever be the explanation, the fact remains that filaments treated as described above are free from gas-sensitivity.

We have found that leakage of air into the envelope of a mass spectrometer over long periods will cause filaments to attain gas-sensitivity even though they are treated as described hereinbefore. Thus the presence of air within a spectrometer at a pressure of. 10- mm. of mercury for a period of 48 hours may cause the filament to become gas-sensitive. If, however, this filament has been subjected to pre-treatment with a hydrocarbon in accordance with the invention, this gas-sensitivity is merely temporary and may be cured by eliminating the air within the envelope and introducing, say a C3, C4 or C5 hydrocarbon 1. e. a hydrocarbon having as its nucleus either 3, 4 or 5 carbon atoms for a considerable period, say 24 hours.

It is desirable, in order to preserve the freedom from gas-sensitivity attained through the treatment of the invention, to maintain a concentration of a C3, C4 or C5 hydrocarbon in the spectrometer during idle periods. Thus if the spectrometer is not in use over a week end, the presence of n-butane within the envelope at a pressure of say 10* mm. of mercury tends to preserve the calibration of the instrument. This treatment with C3, C4 or C5 hydrocarbons will cure gas-sensitivity of a filament temporarily, even though the filament has not been subjected to the pre-treatment described hereinbefore,

itself is begun Thefila ment isheated slowly over a two hour .period y g til a temperature-of ZZQOjKelyin .is attained.

Under the foregoing conditions, :the heating of 5 the filament takes placein the -presence. of sev Thus carbondioxide, carbon .mone oxide, water vapor benzene and-:perhapspther hydrocarbonsaretreleased tromglass and metal I 1 i surfaceswithimthe envelope. In the presence of the oxides of carbon and-the hydrocarbonivapors the filament becomes free fromogas -sensitivity,

Without the pre-treatm ent, however, the cure is only temporary.

We claim: l 1. In the treatment of metallic filaments adapted for service as electron emitters in ionization gauges, the improvement which comprises treattivity of the filament. V 2. Inthe treatment of tun sten filaments'adapt- 1 7 ed for service as electron emitters in'ionization gauges, the improvement which comprises treating the filament with a liquid hydrocarbonand in the presence of a minute amount of the hydrocarbon adheringto the filament as a result of "the prior treatment, thereby reducing gas sensitivity ofthefilamentx adapted for service as electron emitters in ionization gauges, the improvement which comprises treating the filament with a liquid hydrocarbon carbon adhering to thefilament from the prior .,,tre'atment, thereby reducing gas sensitivity of the filament.

5. Inithe: treatment of metallic filaments adapted for service as electron emitters in ioni- 3. m the treatment or metallic--filaments adapted for service as electron emitters in i'onization' gauges, the improvement which comprisestreating the filament with-a liquid aromatic hydrocarbon and subsequently heating it in an evacuated envelope in the presence of a minute amount 7 of the hydrocarbonadheringl to thefilament as a result of the prior treatment,"thereby reducing gas sensitivity of thefilament. i 4. In *the treatment of metallic filaments zation gauges, the improvement which comprises treatingthe' filament with benzene and subsequently heating iteinan evacuated envelope in the presence of a minute proportion of the benz ene adhering to the filament from the prior subsequently heating it in an evacuated envelope g thereby reducing gas sensitivity of the amen In the treatment a of metallic filaments 7 adapted for service as electron emitters in ionivelopein the presencelof-no more than that mi-- nuteproportion of the hydrocarbon which-adheres, V to the filamentinthe prior treatment, the pres v a sure in the envelope being below 5 -10 mm; Hg

during the heating, thereby reducingl ga ii tivity of the filament;

HAROLD WILEY.

HAROLD 'W. WASHBURN; 

